Cigarette filters require a mechanical strength enough to stand a compressive force exerted by smoker's fingers during smoking. Meanwhile, cigarette filters left in natural environments are desirable to be rapidly disintegrated by water or others. Even if illegally dumped, such cigarette filters are rapidly disintegrable and biodegradable, so that environmental pollution can be reduced. Thus a cigarette filter is required to have both a sufficient strength in a dry state and a sufficient disintegration in a wet state.
For example, Japanese Patent No. 3677309 publication (Patent Document 1, JP-3677309B) discloses a cigarette filter material in the form of a sheet having a paper structure and comprising an uncrimped cellulose ester staple fiber and a beaten pulp, wherein the beaten pulp has a degree of beating of Schopper-Riegler freeness of 20 to 90° SR, the uncrimped cellulose ester staple fiber is a staple fiber having an average fiber length of 1 to 10 mm and a fineness of 1 to 10 deniers. This document discloses that in the preparation of the sheet material there may be employed a binder (for example, a water-soluble adhesive) that does not have a bad influence on human body or decrease the taste and palatability of tobacco smoke or the disintegration of the filter material, and that the amount of the binder is preferably as small as possible (for example, not more than 10% by weight in the total weight of the material). In Examples described in this document, there is an example in that a sheet formed from an uncrimped cellulose acetate staple fiber and a beaten pulp by wet paper production process was sprayed with an aqueous solution of a carboxymethyl cellulose in a proportion of 3% by weight on a dry weight basis to give a sheet material.
Japanese Patent Application Laid-Open Publication No. 7-75542 (Patent Document 2, JP-7-75542A) discloses a cigarette filter comprising a tow of a cellulose ester fiber and a water-soluble polymer that is contained in the tow and bonds the fiber, the tow having been processed into a filter rod using not more than 25 parts by weight of water with respect to 100 parts by weight of the tow. Examples of this document include an example that a cigarette filter tip is obtained by adding 5% by weight of a carboxymethyl cellulose sodium salt as a water-soluble polymer to an opened cellulose acetate crimped fiber tow and feeding the opened tow to a wrapping machine to wrap up the opened tow with a filter wrap.
Japanese Patent Application Laid-Open Publication No. 8-322539 (Patent Document 3, JP-8-322539A) discloses a cigarette filter comprising a nonwoven fabric consisting of a cellulose ester composition and a binder having a good water-dispersibility, the nonwoven fabric being wrapped up into a rod form. Examples of this document include an example that a filter plug is produced by blowing a screen wire with a cellulose acetate staple fiber by air flow for lamination or deposition, and spraying the laminate matter on the wire with 10% by weight of a carboxymethyl cellulose using a 5% aqueous solution of a carboxymethyl cellulose, pressing and drying the wet laminate, subjecting the resulting nonwoven fabric to crepe roll treatment, and then wrapping up the fabric.
The cigarette filters containing these materials are disintegrated by water. Unfortunately, the filters do not have a satisfactory disintegration rate and fail to show an immediate disintegration by water. Meanwhile, these documents disclose addition of a carboxymethyl cellulose metal salt for improving a strength of a cigarette filter. With increasing an amount to be added of the carboxymethyl cellulose metal salt, the strength of the filter increases, while the disintegration thereof decreases. Thus a filter having a sufficient strength cannot be disintegrated in a short period of time, and the filter has an insufficient water disintegration. For the reasons, the strength in a dry state is still difficult to be compatible with the disintegration in a wet state.